1. Field of the Invention
The present invention relates generally to waste heat recovery boilers used in combined power plants and, more particularly, to a waste heat recovery boiler which can be adapted to deal with exhaust gases containing sulfur oxides (SOx) as well as those containing no sulfur oxides.
2. Description of the Prior Art
To cope with the rapid increase in electric power demand, thermal power plants having a large capacity have been constructed. In general, the boilers used in such thermal power plant are required to perform a sliding pressure operation in order to attain a high power generation efficiency under the condition of a partial load operation.
An outstanding feature of recent trends in the demand for electric power resides in the fact that an increase in the amount of power generated by atomic power plants enlarges the difference between the maximum and minimum load variations. For this reason, there is a tendency for thermal power plants to be shifted from base load operations to load adjustment operations.
More specifically, in the case of a load adjustment operation, few thermal power plants are operated with boilers operated under the condition of a normally full load. Typical thermal power plants are repeatedly stopped and started while the level of boiler load is being increased and decreased within the range of from 75% load to 50% load to 25% load. That is to say, thermal power plants are commonly used in a so-called daily start stop (hereinafter referred to simply as "DSS") operation to support a medium load. In such DSS operations, thermal power plants are only used during the daytime while electric power demand is high, and at night are stopped; this improves the efficiency of power generation.
For example, combined power plants have recently attracted attention as a highly efficient form of electric power generation. In such a combined power plant, the generation of electric power is performed by first generating electricity by means of a gas turbine, then recovering the heat possessed by the exhaust gases discharged from the gas turbine by means of a waste heat recovery boiler, and then actuating a steam turbine by using the steam generated by the waste heat recovery boiler.
In this manner, the combined power plant utilizes a combination of power generation employing the gas turbine and power generation employing the steam turbine. Accordingly, the combined power plants feature a highly efficient form of power generation as well as the excellent load response which is the characteristic of gas turbines. Therefore, such a combined power plant is capable of coping with the rapid increase and decrease in electric power demand, and hence excels in terms of its capability to follow up load variations. For these reasons, combined power plants are well suited to the DSS operations.
FIG. 13 is a system diagram illustrating the overall system of a typical example of the combined power plant of the prior art.
As illustrated, a fuel air A supplied through an air supply pipe 1 and a fuel F supplied through a fuel supply pipe 2 are mixed and burned in a combustor 3, and the resultant combustion gas causes the rotor of a gas turbine 4 to rotate, thereby generating electric power. After the combustion gas has been utilized in the gas turbine 4, the gas is introduced as an exhaust gas G into an exhaust gas passage 6 of a waste heat recovery boiler 5. A low pressure boiler 10 and a high pressure boiler 15 are arranged along the exhaust gas passage 6 from its downstream side to the upstream side. The low pressure boiler 10 includes a low pressure economizer 7, a low pressure evaporator 8 and a low pressure drum 9, and the high pressure boiler 15 includes a high pressure economizer 11, a high pressure evaporator 12, a high pressure drum 13 and a superheater 14.
Feedwater WF which serves as a fluid to be heated is supplied by a feedwater pump 16 through a feedwater pipe 17 to the low pressure economizer 7. After being preheated to a predetermined temperature in the low pressure economizer 7, the feedwater WF is supplied through a drum feedwater pipe 18 to the low pressure drum 9.
The feedwater WF supplied to the low pressure drum 9 is naturally or forcibly recycled through a low pressure descending pipe 19, the low pressure evaporator 8 and the low pressure drum 9. During this recycling, the feedwater WF is heated and separated into water and steam in the low pressure drum 9. The thus-separated water is further recycled through the low pressure descending pipe 19, the low pressure evaporator 8 and the low pressure drum 9, while the steam is supplied through a main steam pipe 20 to a steam turbine 21.
In the meantime, high temperature water WR which is made to branch off at the outlet of the low pressure economizer 7 is partially supplied to the high pressure economizer 11 through a high pressure feedwater pipe 23 by means of a boiler feedwater pump 22. After being heated to a predetermined temperature in the high pressure economizer 11, the high temperature water WR is supplied through a drum feedwater pipe 24 to the high pressure drum 13.
As with the low pressure boiler 10, the feedwater supplied to the high pressure drum 13 is recycled through a high pressure descending pipe 25, the high pressure evaporator 12 and the high pressure drum 13. During this recycling, the feedwater is separated into water and steam in the high pressure drum 13. The thus-separated steam is supplied through a drum steam outlet pipe 26 to the superheater 14. After the temperature of the steam is further raised in the superheater 14, it is supplied to the steam turbine 21 through a high pressure steam pipe 27.
On the other hand, the water separated in the high pressure drum 13 is recycled through the high pressure descending pipe 25, the high pressure evaporator 12 and the high pressure drum 13. The liquid level of feedwater in each of the high pressure drum 13 and the low pressure drum 9 is controlled by operating a high pressure drum feed valve 28 and a low pressure drum feed valve 29, respectively. In FIG. 13, reference numeral 30 denotes a condenser, and reference numeral 31 denotes a generator.
The steam which has been used for rotation of the rotor of the steam turbine 21 is changed into water in the condenser 30. The resultant water is again supplied as the feedwater WF through the feedwater pipe 17 to the waste heat recovery boiler 5 by means of the feedwater pump 16. However, the feedwater WF in the feedwater pipe 17 has a low temperature of about 34.degree. C. Therefore, if the feedwater WF having such a low temperature is supplied to the low pressure economizer 7, low temperature corrosion will take place in the low pressure economizer 7. For this reason, it is necessary to raise the temperature of the feedwater to a predetermined temperature at which no low temperature corrosion occurs by mixing the feedwater WF with part of the high temperature water WR passing through the high pressure feedwater pipe 23.
For this purpose, part of the high temperature water WR flowing in the high pressure feedwater pipe 23 is supplied from the outlet of the boiler feedwater pump 22 to the feedwater pipe 17 through a recycle channel 33 having a recycle flow regulating valve 32, thereby preventing the occurrence of low temperature corrosion within the low pressure economizer 7.
It is to be noted that the waste heat recovery boiler 5 in the schematic system view of FIG. 13 is shown by way of example as employing as the fuel F a fuel, such as LNG, which exhausts a clean gas containing no sulfur. To cope with the recent trend toward diversification in the kinds of fuel employed, the waste heat recovery boiler 5 may of course employ as the fuel F a dirty oil fuel such as naphtha which contains sulfur.
There are some instances where a waste heat recovery boiler which can be adapted to various kinds of fuel is planned and utilizes the aforesaid structure of the prior art waste heat recovery boiler 5. In such an instance, if the heat transfer area of the low pressure economizer 7 is designed on the assumption that a fuel is used which exhausts a gas containing no SOx (hereinafter referred to as a "clean gas"), it is necessary to raise the temperature of the feedwater at the inlet of the low pressure economizer 7 in order to prevent low temperature corrosion from occurring during the recovery of the heat possessed by the waste gas. This necessity involves the following shortcomings which will be described with reference to FIGS. 14A and 14B.
FIGS. 14A and 14B are characteristic charts in which their horizontal axes uniformly represent the amount of SOx contained in the exhaust gas G, with the vertical axes of FIGS. 14A and 14B representing the temperature of the exhaust gas at the outlet of the waste heat recovery boiler 5 and the temperature of the feedwater, respectively. In FIGS. 14A and 14B, a common vertical line B which corresponds to a clean gas represents the fact that the amount of SOx is zero, while a common vertical line C which corresponds to a dirty gas represents the fact that the amount of SOx is 10 ppm.
Curve D in FIG. 14A represents the temperature of the exhaust gas at the outlet of the low pressure economizer 7 when the heat transfer area of the low pressure economizer 7 is increased, with curve E in the same figure representing the temperature of the exhaust gas at the outlet of the low pressure economizer 7 when the heat transfer area of the low pressure econimizer 7 is reduced.
In FIG. 14B, curve H represents the temperature of the feedwater at the inlet of the low pressure economizer 7, curve I representing the temperature of the feedwater at the outlet of the low pressure economizer 7 when the heat transfer area of the low pressure economizer 7 is increased, curve J representing the temperature of the feedwater at the outlet of the low pressure economizer 7 when the heat transfer area of the low pressure economizer 7 is reduced, curve K representing the steam generation temperature at which steam is generated in the low pressure economizer 7, and a shaded portion L representing the steam generation zone in which steam is generated in the low pressure economizer 7.
As described previously, in order to prevent low temperature corrosion from occurring in the low pressure economizer 7 when either of a clean or dirty gas is exhausted, t is necessary to increase the flow rate of the high temperature water WR flowing in the recycle channel 33 shown in FIG. 13. However, when the flow rate of the high temperature water WR is increased, the temperature of the feedwater at the inlet of the low pressure economizer 7 rises along curve H from point M to point N.
Accordingly, the temperature of the feedwater at the outlet of the low pressure economizer 7 rises from point O to point P as indicated by curve I, and the temperature within the low pressure economizer 7 reaches the steam generation zone indicated as the shaded portion L. This temperature increase results in the problem that the low pressure economizer 7 is damaged owing to the occurrence of adverse phenomena such as steaming, unstable flow, water hammer and so forth.
In addition, the increase in the flow rate of the high temperature water WR involves the drawback of an increase in the capacity of the boiler feedwater pump 22. It is estimated that the pump capacity of the boiler feedwater pump 22 with respect to the dirty gas is about two times greater than that with respect to the clean gas.
On the other hand, when the heat transfer area of the low pressure economizer 7 is reduced from curve I to curve J in order to prevent the phenomenon of steaming in the low pressure economizer 7, the temperature of the feedwater at the outlet of the low pressure economizer 7 falls from point P to point Q, thereby enabling prevention of the phenomenon of steaming. Since the temperature of exhaust gas at the outlet of the waste heat recovery boiler 5 rises from point R to point S, the occurrence of low temperature corrosion can also be prevented.
However, when the clean gas is employed, the temperature of the exhaust gas at the outlet of the waste heat recovery boiler 5 rises from point T to point U in FIG. 14A, and thus in the water heat recovery boiler which is planned on condition that the clean gas is used, the temperature of the exhaust gas at its outlet is raised by about 15.degree. C. Therefore, since a large quantity of heat to be recovered is dissipated in the atmosphere, the aforesaid prior art arrangement is uneconomical from the viewpoint of heat recovery.